Yep.. in addition to potassium and sodium, too little calcium and magnesium in your system will also contribute to cramping.
If you aren't replacing electrolytes when you're active, I'd recommend starting with that.. there are plenty of carb-free options out there and my experience is that they do make a HUGE difference. I've grown fond of nuun since trying it a few years ago when Team Type 1 was using it (and tweeting about it often!) during RAAM, but I really only like a couple of the flavors (tri-berry and lemon-lime).
My potassium and sodium are always at the bottom of the reference range in my lab work - but because I am active my doctor has never been too concerned, but I know personally that I do have to really pay attention to how I stay hydrated or I get cramping (usually in my legs, but sometimes in my arms/hands too). Being hypokalemic will cause cramping, but being hyponatremic can also cause some fairly significant neurologic symptoms as well.. dizzyness, nausea, confusion, headaches, etc... it can be pretty scary if it happens to you, especially because the symptoms are kind of non-specific, and when you have D can easily be attributed to other things.
I pretty much won't drink water on days I am active or outside a lot.. it has to be something with electrolytes. If I skip it, I either get muscle cramps during activity, or charlie horse type cramps afterwards (sometimes in my sleep), which are often really bad.
Sarah T1 dx @ 20 (10/2000)
mom to 3 non-D kids (6, 8, & 13)
with the occasional help of Dexcom